The present invention relates to devices for capturing positional inputs and more specifically to a self-tuning digitizer control circuit.
In general, there are several types of digitizers, including electrostatic and capacitive digitizers. Electrostatic digitizers operate in response to one or more periodic excitation sources selectively energizing a conductive coated substrate. The stylus is coupled to the energized conductive coating. The signal from the stylus is then filtered, processed, and measured to determine the position of the stylus on the substrate. The electrostatic digitizer takes its name from the electrostatic coupling that takes place between the stylus and the conductive coating.
Alternatively, electrostatic digitizers also operate in response to one or more periodic excitation sources energizing the conductive stylus. The stylus is coupled to a conductive coated substrate. Signals from the conductive coating are filtered, processed, and measured to determine the position of the stylus on the substrate.
U.S. Pat. No. 4,853,493 to Schlosser et al., entitled, "Electrographic Apparatus", issued Aug. 1, 1989, discloses an electrostatic digitizer.
Capacitive digitizers operate in response to a periodic excitation source selectively energizing a conductively coated substrate. A conductive stylus (often a finger) with a finite impedance to ground is coupled to the energized conductive coating. The currents required by the excitation source to selectively energize the conductive coating are filtered, processed, and measured to determine the position of the stylus on the substrate.
Filtering and processing is performed by a "front end" section. Both types of digitizers rely on a filter in the front end section for filtering signals related in frequency to the excitation or drive source (or sources) to determine stylus position. The filter must be selective enough to reject unwanted electrical noise which tends to degrade the accuracy of stylus position data. For optimal operation, the filter frequency characteristic must accurately match that of the periodic excitation source. The usual way for accomplishing this match is to tune the filter frequency characteristic to that of the excitation frequency.
Unfortunately, the more selective the filter, the more its frequency characteristic becomes dependent upon the tolerances of the components used to build the filter. The filter frequency characteristic must accurately match that of the periodic excitation source for optimal operation. The usual way of accomplishing this match is to tune the filter frequency characteristic to that of the excitation frequency.
The filter is usually a narrow bandpass filter. The bandwidth must pass the excitation frequency, yet be narrow enough to minimize passage of electrical noise. Unfortunately, the narrower the bandwidth, the more critical it is to match the center frequency to the excitation frequency. Bandpass filters having a predictably stable center frequency are difficult to design because component tolerances tend to detune the center frequency. Widening bandwidth makes the center frequency less critical, but performance suffers due to electrical noise. Using high precision parts adds significant cost. Also, component value drift over time compromises performance. Manually tuning the filter to the excitation frequency adds cost; variable inductors and capacitors are more expensive than non-tunable components and require costly periodic adjustment. Component value drift over time compromises performance.
Therefore, it would be desirable to provide a cost-effective digitizer control circuit which is self-tuning, that is, a digitizer control circuit which controls and automatically retunes the excitation source to match the center frequency of a narrow bandpass filter constructed of relatively low-precision, fixed value components during the life of the digitizer.